1. Field of the Invention
The present invention relates to a high voltage start-up circuit with constant current control, which can be applied to a switching mode power converter, suitable for high voltage power sources of wide input ranges to generate constant current output, so that the output current and time in the start-up circuit can be constant, and the start-up circuit prevents the electronic components from being damaged by excessive output current in case a short circuit occurs at the output end.
2. Description of Related Art
Referring to FIG. 1, a typical high voltage start-up circuit is shown. As illustrated in FIG. 1, the high voltage start-up circuit adapts the negative-bias gate-source voltage of a high voltage junction transistor to restrict the output current. The high voltage start-up circuit comprises a high voltage junction transistor Q1, a first Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) Q2, a second MOSFET Q3, a bias resistor R1, a PNP transistor Q4, a Zener diode ZD1 and a hysteresis comparator COMP1. The drain of the high voltage junction transistor Q1 is coupled to a high voltage power source VIN, the source of the high voltage junction transistor Q1 is coupled to the drain of the first MOSFET Q2, and the gate of the high voltage junction transistor Q1 is directly connected to the reference ground. The source of the first MOSFET Q2 is coupled to the output voltage node VOUT. When the high voltage junction transistor Q1 is turned on, the bias resistor R1 generates a positive bias voltage to turn on the first MOSFET Q2. As soon as voltage level at the output voltage node VOUT reaches a predetermined reference voltage VT, the hysteresis comparator COMP1 generates a control signal to turn on the second MOSFET Q3. Meanwhile, a negative bias voltage below a pinch-off voltage is generated between gate to source of the first. MOSFET Q2 so as to turn off the first MOSFET Q2 and stop outputting current. The Zener diode ZD1 and PNP transistor Q4 are connected in series between the reference ground and the gate of the first MOSFET Q2 to reduce the possibility of abnormal excessively-high voltage generated between the gate and the source of the first MOSFET Q2.
Referring to FIG. 2, another typical high voltage start-up circuit is shown. As illustrated in FIG. 2, the high voltage start-up circuit is similar to the high voltage start-up circuit shown in FIG. 1. The high voltage start-up circuit of FIG. 2 comprises a high voltage junction transistor Q1, a first MOSFET Q2, a second MOSFET Q3 and a bias resistor R1. Wherein the gate of the high voltage junction transistor Q1 is coupled to the gate of the first MOSFET Q2, the drain of the high voltage junction transistor Q1 is coupled to a high voltage power source VIN, the source of the high voltage junction transistor Q1 is coupled to the drain of the first MOSFET Q2, and the source of the first MOSFET Q2 is coupled to an output voltage node VOUT. When the high voltage junction transistor Q1 is turned on, the bias resistor R1 connected between the drain and gate of the first MOSFET Q2 generates a positive bias voltage to turn on the first MOSFET Q2. As soon as voltage level at the output voltage node VOUT reaches a predetermined reference voltage VT, the hysteresis comparator COMP1 generates a control signal to turn on the second MOSFET Q3. Meanwhile, a negative bias voltage is generated between gate and source of the first MOSFET Q2 so as to turn off the first MOSFET Q2 and stop outputting current as well.
FIG. 3 shows a characteristic curve depicting a relationship of the drain current ID and the gate-source voltage Vgs in the high voltage junction transistor mentioned above. In present, the high voltage start-up circuits restrict the output current by using the high voltage junction transistor. Referring to FIG. 3, the increasing of negative bias voltage between gate and source of the high voltage junction transistor leads to the reduction in drain current. However, because the typical high voltage start-up circuit lacks a current detecting resistor, the negative bias voltage between the gate and source of the high voltage junction transistor cannot be maintained and the output current ID is varied from high to low. Also referring to FIG. 4, at the instance of start-up, the high voltage start-up circuit outputs a maximum current IDL (the value of the maximum current depends on the voltage level of the high voltage power source). Then the drain current ID, which is also the output current, is reduced along a parabolic curve as time T goes on. The negative bias voltage of the high voltage junction transistor in the typical high voltage start-up circuit can be expressed as below:Vgs(Q1)=−(Vth(Q2)+Vo)wherein Vth(Q2) is gate to source threshold voltage of the first MOSFET Q2.
As the output voltage Vo rises, gate-source voltage Vgs(Q1) of the high voltage junction transistor Q1 is changed and the output current decreases. In order to provide sufficient output current, it is required to increase current-limiting level of the output current at the instant of start-up. However, in case a short circuit occurs at the output end, it is likely to cause excessive power consumption to burn the circuit.